Optical fibers have many uses in modern communications systems. They serve to direct beams of light between components in such systems. Optical fibers have been developed to have low loss, low dispersion, polarization maintaining properties and can be incorporated into several different types of devices, such as amplifiers, filters, lasers and interferometers. Many communication systems require an isolator.
Fiber optic isolators are passive optical devices that allow light to be transmitted in only one direction. They are most often used to prevent any light from reflecting back down the fiber, as this light could enter the source and cause backscattering or feedback problems. Optical feedback degrades signal-to-noise ratio and consequently bit-error rate. Ideally an isolator would pass all light in one direction and block all light in the reverse direction.
Some of the specifications used to measure a fiber optic isolator include center wavelength, isolation, insertion loss and polarization dependent loss. Center wavelength is the center of the wavelength range in which the isolator is designed to function optimally. This characteristic is usually measured in nm. Isolation, generally measured in decibels (db), is a measure of how effectively back reflections are prevented and the degree to which the isolator can transmit. Insertion loss is the attenuation caused by the insertion of an optical component. Polarization dependent loss is the attenuation caused by polarization variation. Fiber optic isolators are commercially available at various specifications. For example, U.S. Pat. No. 4,893,890, titled “Low-Loss, High-Isolation, Fiber-Optical Isolator,” issued to Lutes on Jan. 16, 1990, and U.S. Patent Publication No. 2004/0169926, titled “Module Assembly for Fiber Optic Isolator,” by Blair, et al., and published on Sep. 2, 2004, both disclose such fiber optic isolators. Both of these references are incorporated in their entirety.
In addition to these optical properties, another physical characteristic, namely size, can be important for some applications. Unfortunately, it is difficult to obtain high performance characteristics in a small size. Accordingly, an improved design is needed.